x86_64: oops_begin() fix

We don't want to see this:

>  BUG: using smp_processor_id() in preemptible [00000001] code: bash/3857
>  caller is oops_begin+0xb/0x6f
>
>  Call Trace:
>  [<ffffffff8020ab4d>] show_trace+0x34/0x4f
>  [<ffffffff8020ab7a>] dump_stack+0x12/0x17
>  [<ffffffff8030d92d>] debug_smp_processor_id+0xad/0xbc
>  [<ffffffff8042388f>] oops_begin+0xb/0x6f
>  [<ffffffff8042520b>] do_page_fault+0x66a/0x7c0
>  [<ffffffff804234bd>] error_exit+0x0/0x84
>

coming out when the kernel is trying to oops.

Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>